What happens to the shaven down teeth that are under the veneers?

Are they not able to be taken care of properly because they are underneath a veneer, and do they start decaying? What happens if one of the now shaven down falls out because its weak from the prep? Doesn't your veneer have to be held on to something? I'm just curious to know what happens to your original teeth once porcelain veneers are placed over them.